Orthogonal frequency division multiplexing (OFDM) has been recently proposed as a multi-access scheme for use in the next generation of cellular wireless systems. A critical issue in the design of these systems is the requirement of strict timing synchronization and access control. In order to maintain orthogonality in the uplink, it is necessary that all signal paths from all mobile stations (“mobiles”) arrive within a so-called receive cyclic prefix of the particular base station that they are attempting to communicate with. Moreover, new mobiles that have not yet synchronized must be able to access the base station without disrupting the uplink data from mobiles already in communication and synchronized with the base station.
A need exists for techniques for construction, detection and estimation of the timing and access signals to be used in OFDM uplink synchronization and access systems. The construction, detection and estimation of timing and access signals is a classic problem, and-involves, in general, a number of standard criteria including timing resolvability, signal bandwidth, the power requirement on the mobile to transmit the signals, and estimation optimality. See, e.g., J. G. Proakis, “Digital Communications,” 3rd ed., New York, McGraw Hill, 1995, which is incorporated by reference herein. However, the design and estimation of signals for certain OFDM multi-access wireless synchronization systems presents a number of unique challenges.
First, the multi-access channel may result in interference between signals. It is desirable to reduce the co-signal interference to improve the signal detection and estimation, and eliminate the need for the computationally difficult procedures such as interference cancellation or multi-user detection and estimation. Also, in wireless systems, signals from different mobiles may be received with widely different powers, and suppression of the co-signal interference is necessary to avoid strong signals from overwhelming weaker ones. This need to suppress co-signal interference is particularly important during access, when new mobiles may not yet be power controlled. Also, when co-signal interference is reduced, mobiles which have failed access can more rapidly increase their transmission power in subsequent access attempts, since the effect of co-signal interference from a high-power mobile is reduced. This potential for faster power increases would reduce the access delay.
Second, in a wireless channel, signal detection and timing estimation should be robust against channel fading, multipath delay spread and any interference appearing in the same band. Moreover, in OFDM systems, the objective of timing estimation is not to resolve all the signal path arrival times. Instead, it is only necessary to determine some timing offset such that all signal paths arrive within the base station cyclic prefix.
Conventional techniques have been unable to address adequately the above-identified concerns relating to the construction, estimation and detection of timing and access signals for synchronization and access control in OFDM systems.